1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a method for transferring partially completed circuits from silicon substrates, to temperature sensitive substrates, for liquid crystal display (LCD) applications.
2. Description of the Related Art
There is broad agreement in the flat panel display (FPD) industry that system-on-glass (SOG) technology is a natural evolutionary step for flat panel displays, especially for mobile devices. In fact, SOG is a natural confluence of display and microprocessor evolution because integration is a proven solution for greatly reducing costs, while improving the compactness and reliability of electrical systems.
Display modules have received some enhanced functionality, like display drivers and analog-to-digital converters, thanks to low-temperature polysilicon (LTPS) technology. Conventionally, the best means for achieving adequate LTPS performance for SOG devices at a competitive cost is the crystallization of a thin amorphous layer of silicon with a laser beam. Unfortunately, this approach remains relatively expensive, even years after being introduced to LCD production. Moreover, even if polysilicon thin film transistors (TFTs) can be efficiently produced using this method, it is unlikely that they have sufficient capability to realize some sophisticated functions like CPU operations, digital driver LCDC, digital-to-analog converter (DAC), memory, graphical controller, wireless, MPU, and digital signal processing. Finally, the steadiness of drive currents produced by poly-Si TFTs may be inadequate for new technologies such organic electroluminescent displays.
There are two major problems that prevent the integration of the above-mentioned functions into integrated LCD products. One is the film quality, and the other is the design limitations associated with glass substrates. The poly-Si created by laser annealing consists of numerous small grains, with a typical size of less than 1 micron. Each grain is oriented differently and surrounded by grain boundaries, which cause degradation in the resultant device characteristics. Further, the poly-Si TFT device characteristics are not uniform. The problem with the small, non-uniform grain size is compounded by the fact the energy-producing operations that can address this problem are limited by the sensitivity of LCD substrates to high temperatures.
In short, the convolution between cost and performance of polysilicon devices still pales compared to that of single-crystal silicon (c-Si). The trouble is in creating quality thin films of c-Si on transparent or flexible substrates, which usually are made of materials other than silicon. Other solutions to this hybrid field problem include Fluidic-Self-Assembly™ (FSA) by Alien Technology. FSA works fairly well for plastic substrates and semiconductor blocks thicker than 50 microns. Unfortunately, FSA placement depends on random probability and gravity. Because the probability of successful placement is small (<<20%), a large amount of blocks are needed. Also, as the blocks get smaller, Brownian motion becomes more disruptive to precise placement and more time is required for settlement. Finally, if glass substrates are desired, then another problem is the efficient etching of precise-sized holes. Other rapid assembly techniques, like capillary self-assembly, still require fluid, which usually demands use of surfactants, and remain susceptible to Brownian motion.
SOITEC and other researchers have developed and refined a means of efficiently creating thin films of c-Si by ion-cutting with a high dose hydrogen implantation. In related work, Joly et al. have extended the ion-cutting process (Smart-Cut) to produce devices on one substrate, and transfer these devices to a different substrate. While their work describes a process for transferring the devices, there is little discussion regarding the impact of high dose hydrogen implantation on device performance. It is acknowledged by many that the required large doses of hydrogen (˜5e16 atoms/cm2) can result in highly defective regions in the transferred silicon films.
FIG. 1 is a diagram of a hydrogen-induced cleaving process using a hydrogen blocking mask (prior art). To address some of the problems associated with the use of hydrogen, Bruel et al., describe the use of a blocking mask to protect active silicon regions from damage during the hydrogen implant. A blocking mask adds an extra step to the fabrication process. However, if hydrogen is allowed to diffuse into the substrate near the channel areas, then dopants in the channel and source/drain regions may bind with the hydrogen and impact the performance of the transistor. Therefore, conventional processes have not proved to be practical for large-scale fabrication processes, or for the transfer of very large active Si areas, such as VLSI circuits with a plurality of blocked areas.
It would be advantageous if an improved substrate cleaving processes were developed that minimized the detrimental effects of hydrogen.